1. Field of the Invention
The present invention relates to engine knock detection. More specifically, the present invention relates to methods and apparatus for digital knock detection in internal combustion engines.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Engine knock detection and control systems are employed to improve the performance of internal combustion engines. The engine knock phenomenon is manifested as a chaotic disturbance that occurs during the combustion cycle in which burning of the fuel/air mixture proceeds in a violent unstable manner. The violent burning of the fuel/air mixture is typically accompanied by combustion chamber shock wave formation and resultant high amplitude pressure perturbations. The perturbations of combustion chamber pressure occur during the fuel burning stage and after the occurrence of spark ignition.
The high amplitude pressure perturbations can produce annoying audible "knock" noise that is very distinguishable at low engine RPMs. At higher engine RPMs, the knock can be disguised by other engine noises. The knock phenomenon is characterized by amplitude fluctuations of the combustion chamber pressure which generally exceed one bar peak-to-peak and occurs at acoustic vibration frequencies within the range of (4-12) kHz.
When knock detection and control is ineffective, damage to the engine can occur in the presence of heavy knock. Additionally, ineffective knock detection results in false alarm rates and missed detections at lower knock levels. A false alarm is an inadvertent adjustment to the spark ignition control circuitry intended to compensate for a perceived knock event that did not actually occur. A missed detection is the failure of a knock detector to discern an actual knock event which generates a combustion chamber pressure exceeding one bar peak-to-peak amplitude. Further, the knock phenomenon results in less than optimum fuel burning, loss of combustion efficiency and an increase in engine emission levels.
In contrast to the knock event is a no-knock event. The no-knock event is manifested as a slight disturbance that occurs during the burning of the fuel/air mixture of the combustion cycle. The burning of the fuel/air mixture is accompanied by low amplitude combustion chamber pressure perturbations. The no-knock phenomenon is characterized by slight amplitude fluctuations of the combustion chamber pressure occurring at acoustic vibration frequencies within the range of (4-12) kHz and which do not exceed one bar peak-to-peak amplitude.
The knock and no-knock events within an internal combustion engine are also characterized by knock sensor vibration signature patterns. The vibration signature patterns of knock and no-knock events are distinguishable from one another. Typically, the vibration signature patterns of knock and no-knock events are analyzed to assist in designing a knock detector. A knock detector is employed to detect the knock phenomenon and to provide a signal to an engine control module to adjust, for example, the spark ignition mechanism. Changes in the engine noise level within an internal combustion engine are utilized to detect a knock event. The vibration signature patterns of knock and no-knock events can be analyzed utilizing a time-frequency analyzer. The time-frequency analyzer generates plots of vibration amplitude versus time and frequency which exhibit stochastic or quasi-repetitive patterns due to the variable conditions existing within the combustion chamber.
The objective in utilizing a knock detector is to eliminate the knock events within an internal combustion engine as they are annoying and may potentially cause engine damage. Contemporary American and foreign automobiles provide engine knock detection by means of electronic circuitry which continually processes signals from one or more sensors mounted on the engine block or head. The sensor is a transducer that converts mechanical vibration to an electrical signal. The electrical signal is directed to a filter to limit the sensor output signals to the (4-12) kHz range of interest. Based upon empirical evidence, a signal threshold level is established. If the filtered sensor output signal exceeds the threshold level, a knock event is deemed to have occurred. The detection of the knock event, results in an automatic adjustment to the ignition timing within the engine distributor. By adjusting the ignition timing, the probability of a subsequent knock event occurring is greatly reduced as is known in the art. However, the ignition timing adjustment affects all cylinders without regard to which cylinder was actually the source of the knock event. The vibration sensor is sensitive to spurious noises in addition to actual knock events resulting in unnecessary ignition timing adjustments (e.g., false alarms).
Several recent approaches incorporate the use of digital knock detection devices. The first device performs frequency domain analysis on the time history of the vibration sensor wave pattern. The frequency analysis is utilized to determine a multi-spectrum knock index that is used to compensate for background noise. The multi-spectrum knock index is based upon a set of equations using weighted averaging of previous frequency peak values when knock does not occur. Unfortunately, this approach appears to be susceptible to false alarms produced by spurious engine noises such as valve closings and piston slaps which generate harmonics in the same frequency range as a knock event. This device is more fully described in the Society of Automotive Engineers Publication No. SAE 920702 entitled Engine Knock Detection Using Multi-Spectrum Method by M. Kaneyasu et al., reprinted from Sensors and Actuators 1992 (SP-903), February, 1992.
The second approach to knock detection and control utilizes a narrowband sensor with the detection signal processed over the window of from (10 to 90) crankshaft degrees after top dead center (ATDC). A gain constant K, which characterizes the knock detection threshold, is calculated based upon breakpoints in cumulative probability curves, relative to a logarithmic scale for the peak detector output. However, the data presented does not indicate how resistant this second approach is to engine noise sources at high RPM. This device is more fully described in the Society of Automotive Engineers Publication No. SAE 891964 entitled A New Method to Automatically Optimize the Knock Detection Level in the Knock Control System by T. Iwata et al., September, 1989.
The third approach features a knock detection strategy which includes wideband filtering, window energy estimation and standard deviation analysis. This approach uses signal energy from a sliding window but does not compensate quickly to changes in the engine noise floor due to valve closings, piston slaps or other interference sources. This approach is more fully described in the Society of Automotive Engineers Publication No. SAE 910858 entitled Different Methods of Knock Detection and Knock Control by K. Schmillen et al., reprinted from Sensors and Actuators 1991 (P-242), February, 1991.
Thus, there is a need in the art for improvements in digital knock detectors for spark ignition systems which control the ignition timing on an individual cylinder basis and are not susceptible to spurious noises at high engine RPMs resulting in unnecessary ignition timing adjustments.